Engine braking method and system

ABSTRACT

During engine braking of a turbocharged internal combustion engine, the exhaust gas pressure increases and this is used to pressurize the seals between the turbocharger shaft and the bearing housing so as to prevent oil leakage into the compressor housing. Immediately after engine braking, stored exhaust gas pressure is used to pressurize the seals at the turbine end so as to prevent oil leakage into the turbine housing. In an alternative arrangement the exhaust gas is used to generate a reduced pressure in the bearing housing to increase the pressure gradient across the seals.

The present application claims priority to Great Britain PatentApplication No. 0814764.7 filed Aug. 13, 2008, which is incorporatedherein by reference.

The present invention relates an engine braking method and system for aninternal combustion engine. It also relates to a method for reducing oilleakage in a turbocharger.

Engine or exhaust braking systems of various forms are widely used invehicle engine systems, particularly in relation to diesel engines usedto power large vehicles such as trucks. The engine brake systems may beemployed to enhance the effect of the conventional friction brakesacting on the vehicle wheels or, in some circumstances, may be usedindependently of the normal wheel braking system, for instance tocontrol down hill speed of a vehicle. With some engine brake systems,the brake is set to activate automatically when the engine throttle isclosed (i.e. when the driver lifts his foot from the throttle pedal),and in others the engine brake may require manual activation by thedriver, such as depression of a separate brake pedal.

In one form of conventional engine brake system an exhaust valve in theexhaust line is controlled to block substantially the engine exhaustflow path when braking is required. This produces an engine brakingtorque by generating a high backpressure that acts on the engine pistonsduring the exhaust stroke. U.S. Pat. No. 4,526,004 discloses such anengine braking system for a turbocharged engine in which the exhaustvalve is provided in the turbine housing of a fixed geometryturbocharger.

Turbochargers are well known devices for supplying air to the intake ofan internal combustion engine at pressures above atmospheric (boostpressures). A conventional turbocharger essentially comprises an exhaustgas driven turbine wheel mounted on a rotatable shaft within a turbinehousing. Rotation of the turbine wheel rotates a compressor wheelmounted on the other end of the shaft within a compressor housing. Thecompressor wheel delivers compressed air to the engine intake manifold.The turbocharger shaft is conventionally supported by journal and thrustbearings, including appropriate oil lubricating systems, located withina central bearing housing connected between the turbine and compressorwheel housing. Oil is supplied from the engine and allowed to drain fromthe bearing housing back to the oil sump in the engine crankcase. It isimportant to provide an effective sealing arrangement at each end of therotating shaft to prevent oil leakage from the central bearing housinginto the compressor or turbine housing.

At the compressor end of the turbocharger, during its normal operation,the sealing arrangement has to be able to withstand the increasinglyhigh boost pressures that are delivered by modern turbochargers. Thepressure of the bearing housing is effectively at the same pressure asthe engine oil sump (typically around 100 millibar) and there is thus apressure gradient between the bearing housing and the compressor housingwhich prevents the leakage of lubrication oil from the bearing housinginto the compressor housing. The sealing arrangement typically comprisesone or more ring seals arranged between the shaft and the bearinghousing and received in respective grooves, in the manner of pistonrings. The seals are arranged with a radial clearance so as to allow thepassage of gas in small volumes across the seals but to choke the flowso to accommodate the pressure drop.

At the turbine end of the turbocharger there is a similar sealingarrangement using piston ring seals. Although oil pressure in thebearing housing varies, it will generally be higher than the pressure inthe turbine housing behind the turbine wheel (this is the back side ofthe turbine wheel which is proximate the bearing housing). During normalengine-fired mode the pressure drop from the bearing housing to theturbine housing is such that there can be a risk of oil leakage from thebearing housing into the turbine housing, and thus into the exhaust gasflow. This is undesirable as the high temperatures within the turbinecauses the oil to coke and deposits build up within the housing whichadversely affect the turbine performance. The presence of the sealsmitigates this risk.

Using an exhaust valve to effect engine braking can be problematic in aturbocharged engine. In particular, when the valve is substantiallyclosed so as to effect engine braking by creating the back pressure thatbrakes the engine, the restriction of the exhaust flow means that therotational speed of the turbine of the turbocharger is reducedsignificantly. The compressor wheel thus rotates at a correspondinglylow speed with the result that the compressor boost air pressuredelivered to the engine is significantly reduced. At low speeds the low(or even negative) boost pressure at the compressor end can drop belowthe pressure in the bearing housing, particularly when the pressures inthe bearing housing are elevated by crankcase gas pressure. As a resultoil is able to leak along the turbocharger shaft in the bearing housing,past the seals and into the compressor housing past the seals,particularly if engine braking is used for extended periods of time. Thereduction in rotational speed of the shaft means that pressure behindthe turbine wheel increases which exacerbates the tendency of the oil totravel along the shaft to the compressor housing. Leakage of oil intothe compressor housing is undesirable as it contaminates the pressurisedair entering the engine intake manifold.

Immediately after the engine brake is released and normal engine firedmode is resumed, there is a short period during which the shaftaccelerates to a steady state. In this period the pressure in theturbine housing is relatively low and there is a risk of oil leakinginto the turbine housing from the bearing housing. This is undesirablefor reasons mentioned above.

An alternative approach to engine braking is to use a compression brakewhich operates to modify operation of the engine valves in such a mannerthat the compressed air in the engine cylinders is allowed to escapewhen the engine throttle is closed so that the air cannot be used togenerate power for the vehicle. However, compression brakes arerelatively complex and expensive.

In turbocharged engines it is possible to achieve engine braking byemploying a variable geometry turbocharger instead of using an exhaustvalve. Variable geometry turbines differ from fixed geometry turbines inthat the size of a turbine exhaust gas inlet passageway can be varied tooptimise gas flow velocities over a range of mass flow rates so that thepower output of the turbine can be varied to suit varying enginedemands. In order to achieve engine braking, the inlet passageway maysimply be closed to its minimum flow area when braking is required andthe level of braking may be modulated by control of the inlet passagewaysize by appropriate control of the variable geometry.

It is one object of the present invention, amongst others, to obviate ormitigate the aforementioned disadvantages. It is also an object of thepresent invention to provide for an alternative or an improved enginebraking system. It is also an object of the present invention to providefor an improved or alternative method of reducing oil leakage in aturbocharger.

According to a first aspect of the present invention there is providedan engine braking method for an internal combustion engine having an airintake path and an exhaust gas path, a turbocharger comprising a turbinein the exhaust gas path and a compressor in the intake path, the turbinecomprising a turbine wheel rotatably disposed in a turbine housing, thecompressor comprising a compressor impeller rotatably disposed in acompressor housing, the turbine wheel and compressor impeller beingconnected by a turbocharger shaft, a bearing housing disposed betweenthe compressor and turbine for housing a bearing assembly to support theturbocharger shaft in rotation, at least one first seal between acompressor end of the shaft and the bearing housing, the methodcomprising: operating the engine in an engine braking mode by activatingan exhaust gas brake to impede the flow of exhaust gas in the exhaustgas path when the engine throttle is closed, thereby generating anexhaust gas back-pressure in the exhaust gas path; diverting at least aportion of the exhaust gas during engine braking mode from the exhaustgas path in order to increase the pressure gradient across the at leastone first seal in the direction from the bearing housing to thecompressor housing.

Increasing the pressure gradient may be achieved by using the divertedexhaust gas to increase the pressure on a compressor side of the atleast one first seal or to decrease the pressure on a bearing housingside by, for example, using it to decrease the pressure directly, orindirectly, within the bearing housing. In each case the pressuregradient across the seal is designed to ensure that leakage of lubricantfrom the bearing housing across the at least one first seal and into thecompressor housing is prevented or restricted. It will be understoodthat increasing the pressure gradient may include reducing a negativepressure gradient (to a less negative value or to a positive value) orsubstantially equalising the pressures on each side of the at least onefirst seal.

The exhaust gas may be diverted into a passage in the turbocharger thatemerges to one side of the at least one first seal. The passage may passthrough the bearing housing.

The exhaust gas may be directed to one side of the at least one firstseal to increase the pressure on that side so as to increase thepressure gradient across the seal.

The at least one first seal may be a pair of axially spaced first sealsin which case the diverted exhaust gas may be directed to a spacebetween the seals.

The at least one first seal may be disposed at a bore in compressor backplate and the exhaust gas may be directed through a passage in the backplate. Alternatively, the passage may emerge through the bearing housingso as to direct exhaust gas at a back face of the compressor wheel.

The exhaust gas brake may be provided by an exhaust brake valve in theexhaust gas path, preferably downstream of the turbine wheel. It may bedisposed in the outlet of the turbine or downstream thereof.

Alternatively, the exhaust gas brake may be provided by a variablegeometry turbine in which a movable member is moved to reduce the sizeof an inlet passageway of the turbine so as to restrict the exhaust gasflow through the turbine and create the back-pressure.

The exhaust gas may be diverted to a bleed path during engine brakingmode. The bleed path may include a plenum chamber for the temporarystorage of pressurised exhaust gas. The exhaust gas may be permitted toflow out the plenum chamber immediately after operation of the engine inengine braking mode to maintain the favourable pressure gradient acrossthe at least one first seal for a short period. The plenum chamber maybe selectively put in fluid communication with at least one second sealprovided between a turbine end of the shaft and the bearing housing,preferably after operation of the engine in the engine braking mode soas to provide a favourable pressure gradient across the at least onesecond seal.

The bleed path may include a valve, the valve being operated to closethe bleed path when the engine is not being operated in engine brakingmode.

The diverted exhaust gas may be filtered in the bleed path.

The increased pressure gradient may be achieved by delivering thediverted exhaust gas into a conduit in fluid communication with thebearing housing. For example, the conduit may be connected, directly orindirectly, to a crankcase ventilation port of the engine at a locationdownstream of the port, the exhaust gas being passed through a jet pumparrangement so as to effect a decrease in pressure in the conduit, thepressure in the engine crankcase and therefore the bearing housing ofthe turbocharger.

According to a second aspect of the present invention there is providedan engine braking system comprising: an internal combustion engine withan air intake path and an exhaust gas path; a turbocharger comprising acompressor for delivering pressurised air to the air intake path and aturbine for receipt of the exhaust gas from the engine, the turbinecomprising a turbine wheel rotatably disposed in a turbine housing, thecompressor comprising a compressor impeller rotatably disposed in acompressor housing, the turbine wheel and compressor impeller beingconnected by a turbocharger shaft, a bearing housing disposed betweenthe compressor and turbine for housing a bearing assembly to support theturbocharger shaft in rotation, at least one first seal between theshaft and a compressor end of the bearing housing; an exhaust gas brakein the exhaust gas path and operable to effect engine braking, anexhaust gas bleed path extending from the exhaust gas path to a locationbetween the at least one first seal and the compressor housing fordiverting at least a portion of the exhaust gas from the exhaust gaspath during engine braking

The at least one first seal may comprise an annular seal which may besplit and may be in the form of a piston ring. It may be received in agroove defined in the shaft or a boss or other component fixed to, orintegrally formed with, the shaft. The at least one first seal may besupported in a bore in the bearing housing, including, for example, in abore defined in a compressor back plate disposed between the compressorand an interior of the bearing housing.

According to a third aspect of the present invention there is providedan engine braking system comprising: an internal combustion engine withan air intake path and an exhaust gas path; a turbocharger comprising acompressor for delivering pressurised air to the air intake path and aturbine for receipt of the exhaust gas from the engine, the turbinecomprising a turbine wheel rotatably disposed in a turbine housing, thecompressor comprising a compressor impeller rotatably disposed in acompressor housing, the turbine wheel and compressor impeller beingconnected by a turbocharger shaft, a bearing housing disposed betweenthe compressor and turbine for housing a bearing assembly to support theturbocharger shaft in rotation, at least one first seal between theshaft and a compressor end of the bearing housing; an exhaust gas brakein the exhaust gas path and operable to effect engine braking, anexhaust gas bleed path extending from the exhaust gas path to a jet pumpin fluid communication with the bearing housing.

The engine may further comprise a crankcase with a ventilation port forthe venting of gas, the jet pump being disposed downstream of thecrankcase ventilation port, preferably in a conduit connected thereto.

According to a fourth aspect of the present invention there is provideda method for reducing oil leakage in a turbocharger comprising acompressor for delivering pressurised air to an air intake path of aninternal combustion engine, and a turbine for receipt of the exhaust gasfrom the engine, the turbine comprising a turbine wheel rotatablydisposed in a turbine housing, the compressor comprising a compressorimpeller rotatably disposed in a compressor housing, the turbine wheeland compressor impeller being connected by a turbocharger shaft, abearing housing disposed between the compressor and turbine for housinga bearing assembly to support the turbocharger shaft in rotation, atleast one first seal between a compressor end of the shaft and thebearing housing, activating an exhaust gas brake to impede the flow ofexhaust gas into the turbine and diverting at least a portion of theimpeded exhaust gas flow in order to increase the pressure gradientacross the at least one first seal in the direction from the bearinghousing to the compressor housing.

According to a fifth aspect of the present invention there is provided amethod for reducing oil leakage in a turbocharger comprising acompressor for delivering pressurised air to an air intake path of aninternal combustion engine, and a turbine for receipt of the exhaust gasfrom the engine, the turbine comprising a turbine wheel rotatablydisposed in a turbine housing, the compressor comprising a compressorimpeller rotatably disposed in a compressor housing, the turbine wheeland compressor impeller being connected by a turbocharger shaft, abearing housing disposed between the compressor and turbine for housinga bearing assembly to support the turbocharger shaft in rotation, atleast one seal between a turbine end of the shaft and the bearinghousing, activating an exhaust gas brake to impede the flow of exhaustgas into the turbine and diverting at least a portion of the impededexhaust gas flow into a plenum chamber for storage therein, andfollowing deactivation of the engine gas brake releasing the storedexhaust gas flow and directing it to the at least one seal in order toincrease the pressure gradient across the at least one seal in thedirection from the bearing housing to the turbine housing.

This method may be used in conjunction with any of the aforementionedmethods. In particular the plenum chamber may also supply stored exhaustgas to a compressor end seal or seals after engine braking in additionto exhaust gas being supplied during engine braking.

Specific embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 is a diagrammatic representation of an engine braking system ofthe present invention;

FIG. 2 is a longitudinal sectioned view through part of a turbochargerin accordance with the present invention;

FIG. 3 is a longitudinal section view through an alternativeturbocharger embodiment in accordance with the present invention; and

FIG. 4 is a diagrammatic representation of an alternative embodiment ofthe present invention.

Referring to FIGS. 1 and 2, an internal combustion engine 1 usingcompression ignition has an inlet manifold 2 for the introduction of airvia an air inlet path 3 and an exhaust manifold 4 for the expulsion ofexhaust gases to an exhaust gas path 5. The engine 1 is turbocharged bya turbocharger 6 comprising a turbine 7 in the exhaust gas flow path 5and a compressor 8 in the air inlet path 3. An exhaust brake valve 9 isdisposed downstream of the turbine 7 in the exhaust gas flow path 5 andin operation can be used to restrict selectively the flow of exhaust gasin the path 5 and apply engine braking.

The compressor 8 and turbine 7 are interconnected by a commonturbocharger shaft 10 that is supported for rotation in a bearinghousing 11 disposed between housings 12, 13 (see FIG. 2) of thecompressor 8 and turbine 7 respectively. In operation, exhaust gasenters the turbine housing 13 where it rotates a turbine wheel 14 (FIG.2), which in turn rotates a compressor wheel 15 (FIG. 2) of thecompressor 8, the compressor impeller wheel 15 drawing intake air fromatmosphere and delivering boost air to the inlet manifold 2 of theengine 1 via an intercooler 16 (FIG. 1).

During engine braking i.e. when the exhaust brake valve 9 issubstantially closed, exhaust gas is bled from the exhaust gas flow path5 at a suitable position and is selectively directed along bleed path 17to seals 18, 19 (FIG. 2) between the turbocharger shaft 10 and thebearing housing 11 via a bleed valve 20. A first pair of seals 18 isdisposed immediately adjacent to the compressor 8 and a second pair ofseals 19 is disposed immediately adjacent to the turbine 7. In each casethe seals 18, 19 are designed to help maintain a pressure differentialbetween the relatively low pressure environment of the bearing housing11 and the potentially high pressure environment of the compressorhousing 12 and the low pressure at the back face of the turbine 7 in theturbine housing 13 during operation of the engine in a normal firedmode. This prevents bearing oil (or other lubricant) leaking along theturbocharger shaft 10 across the seals 18, 19 and into the compressor orturbine housings 12, 13. When the engine is operated in braking mode,the back-pressure generated serves to reduce the rotation speed of theturbocharger significantly and, as a consequence the compressor boostpressure drops such the pressure difference across the seals 18 is notso marked or even reverses in direction. The supply of exhaust gas (air)to the seals 18 via path 17 during engine braking thus serves tomaintain a pressure difference such that leakage of oil into thecompressor housing 12 is prevented or significantly restricted.Similarly exhaust gas (air) can be supplied to the turbine end seals 19,if required, so as to prevent leakage of oil into the turbine housing13. In practice this may only be required for a short period immediatelyafter engine braking when the engine resumes in normal fired mode.

The valve 20 is operable to permit exhaust gas to flow along path 17 tothe seals 18 only when the exhaust brake valve 9 is closed so as toapply engine braking. Conversely, the valve 20 is closed to prevent thepassage of the exhaust gas to the seals 18 generally when the exhaustbrake valve 9 is open (i.e. during engine-fired mode). The valve 20 maybe an electrically-operated valve under the control of the enginemanagement system or otherwise. It may be biased closed by a suitablebiasing means such as a spring or the like and designed such that oncethe pressure in the path rises to engine braking levels the valve isopened automatically against the action of the spring. A filter 21(FIG. 1) is used to remove carbon and/or other particulates from theengine braking gas before it reaches the seals 18, 19. An optional gasplenum chamber 22 (which can be any sort of pressure vessel includingpart of an accumulator or the like) is provided to retain a temporarystorage of the gas. The chamber 22 allows for pressurised gas to bedelivered to one or both of the seals 18, 19 for a short period of timeafter the valve 20 is closed so as to maintain the pressure at the sealsfor the short time period after the engine braking mode is over (andwhen normal fired mode resumes) and the turbocharger shaft 10accelerates to a steady state condition. This may be particularlydesirable in relation to the turbine end seals 19 for reasons discussedabove and on this basis, in one embodiment, the plenum chamber 22 isonly employed in a dedicated path to the turbine end seals 19. A checkvalve 22 a, or other one-way valve, is provided at the plenum chamber toprevent air escaping from the chamber 22 back into the exhaust passage.

The application of exhaust gas to improve the sealing of the shaft 10during exhaust braking takes advantage of the fact that in a dieselengine 1 the gas flowing in the exhaust gas flow path 5 during enginebraking is essentially clean air. This is because in a diesel engine airand fuel are delivered into the engine combustion cylinders separatelyand when the driver of a vehicle with a diesel engine closes the fuelthrottle (i.e. when the driver lifts his foot from the throttle pedal)air still flows into the engine cylinders where it is compressed andexhausted despite the fact that no fuel is delivered.

The relevant parts of the turbocharger are shown in more detail in FIG.2. The turbine wheel 14 is rotatably disposed with the shaft 10 in theturbine housing 13 and, similarly, the compressor impeller 15 rotateswith the shaft 10 in the compressor housing 12. The turbine wheel 14 andcompressor impeller wheel 15 are mounted on opposite ends of the commonturbocharger shaft 10, which is rotatable on a pair of journal bearings23 in the bearing housing 12 connected between the compressor andturbine housings 12, 13.

The compressor end of the shaft 10 supports a thrust bearing assembly 24that interacts with an oil seal assembly rotatable with the shaft 10.The oil seal assembly comprises an oil slinger 25 with an integral axialboss 26 about which the pair of seals 18 is disposed immediatelyadjacent to the compressor impeller 15. The seals 18 are in the form ofaxially spaced, split piston rings disposed in annular grooves in theboss 26 which extends through a bore in a compressor back plate 27 thatserves as an end wall of the bearing housing 11 separating it from thecompressor housing 12. The seals 18 are supported on a circumferentialsurface that defined the bore through the back plate 17 and extend partway into the grooves thereby providing a labyrinthine sealingarrangement that is known in the art. Oil is delivered through agenerally radial passage in the thrust bearing assembly 24 forlubricating the same and the oil slinger 25 is designed to prevent theoil reaching the compressor-end seals 18. The slinger 25 typicallycomprises a number of radially extending passages, which effectively actas vanes for slinging oil away from the shaft 10 and in particular awayfrom the compressor-end seals 18 and the compressor housing 12. Detailsof the compressor end bearings and oil slinger are not important to anunderstanding of the present invention and will not be describedfurther. Oil is supplied to the bearing housing 11 from the oil systemof the internal combustion engine 1 via oil inlet 28 and is fed to thejournal bearings 18,19 and the thrust bearing 24 by oil passageways 29.It drains from the bottom of the bearing housing 11 into the oil sump ofthe engine crankcase.

The turbine wheel 14 is joined to the turbine end of the turbochargershaft 10 at a seal boss 30. Generally the seal boss 30 is formedintegrally with the shaft 10 (and as such forms part of the shaft) andis joined (for instance by friction welding) to a boss portion on theback of the turbine wheel 14. The seal boss 30 extends through a bore 31in a wall 32 of the bearing housing 11 and into the turbine housing 13.The seal boss 30 is sealed with respect to a circumferential surface ofthe wall 32, that defines the bore 31, by the axially spaced seals 19,each in the form of a piston split ring. The seals 19 extend radiallyfrom the wall 32 surface into grooves defined in the boss 30 but leave aradial clearance so as to provide a labyrinthine sealing arrangementthat inhibits oil and gas leakage through the bore 31, as is known inthe art.

A pressed metal heat shield 33 is located in the turbine housing 13between the turbine wheel 14 and the bearing housing 11. The heat shieldseparates the hot exhaust gas flow from the bearing housing to reduceheat transfer to the bearing housing which could otherwise result inoverheating of the bearings 23.

The air path 17 from the exhaust path 5 to the compressor end seals 18is defined in part by a passage 35 through the bearing housing 11 andthe compressor back plate 27, although other routes through the bearinghousing 11 may be possible. The outer edge of the back plate 27 issealed at 34 to the bearing housing in the region around the passage 35.At the turbine end, the air path is defined in part by a passage 36through the bearing housing 11 and the wall 32, and emerges into thebore 32 between the seals 19.

The pressurised air developed during engine braking is delivered to alocation between each seal in the compressor end seal pair 18 via theair path 17 including passage 35. Here the air provides a positivepressure gradient (or reduces the unfavourable negative pressuregradient) across the axially outermost seal so as to prevent or restrictoil (or other contaminants) progressing along the shaft 10 from thebearing housing 11 to the compressor housing 12 past the seal 18. Evenif the raised pressure between the seals remains below that within thebearing housing 11 in the region of the seal, the pressure differenceacross the seals may nevertheless be reduced and so restrict oilleakage. However, it is preferable that the pressure within the space isat a level at least generally approximate the maximum pressure that willoccur in the bearing housing 11 adjacent to the seals 18, and morepreferably still higher than this level.

When the engine brake valve 9 is opened to so as to allow resumption ofnormal engine fired mode the exhaust gas is directed into at least thepassage 36 so as to apply a favourable pressure gradient across theturbine end seals 19 thereby preventing or restricting oil entering theturbine housing 13 from the bearing housing 11.

It is to be appreciated that the passage 36 may be omitted in some ofthe embodiments so that only the compressor end seals 18 arepressurised. Alternatively, a suitable valve may be provided in the path17, which is operable to direct the exhaust gas pressure generatedduring engine braking just to the compressor end seal pair 18 and toallow gas to flow to the turbine end seal pair 19 at the end of enginebraking.

FIG. 3 shows an embodiment in which the turbocharger 6 has a variablegeometry turbine 7. In this embodiment the exhaust brake valve 9 may beomitted on the basis that exhaust braking is provided by closing downthe turbine inlet as will be described below. For convenience and easeof understanding, the same reference numerals have been used in FIG. 3for parts common to FIGS. 1 and 2.

The turbine housing 13 defines a volute or inlet chamber 40 to whichexhaust gas from the exhaust manifold 4 of the internal combustionengine 1 is delivered. The exhaust gas flows from the inlet chamber 40to an outlet 41 via an annular inlet passageway 42 defined on one sideby a radial wall 43 of a movable annular member 44, and on the otherside by a facing radial shroud plate 45 of the housing 13. Acircumferential array of nozzle vanes 46 extends across the inletpassageway 42 from the radial wall 43, the vanes 46 and wall 43 beingcollectively referred to in the art as the “nozzle ring”. The shroudplate 45 is perforated by slots through which the vanes 46 are movableinto an annular cavity 45 a defined in the bearing housing.

Exhaust gas flowing from the inlet chamber 40 to the outlet 41 passesthrough the inlet passageway 42 and over the turbine wheel 14, which, asa result, drives the compressor impeller wheel 15 via the turbochargershaft 10.

The moveable member 44 of the variable geometry turbine comprises notonly the radial wall 43 but also axially extending outer and innerannular flanges 48, 49 that extend from an outer end of the radial wall43 into an annular cavity 50 provided in the turbine housing 13. Withthe turbine construction shown in the figures, the majority of thecavity 40 is in fact defined by the bearing housing 11. This is purelyas a result of the construction of the particular turbocharger to whichthe invention is in this instance is applied and for the purposes of thepresent invention no distinction is made between the turbine housing andbearing housing in this regard. The cavity 50 has a radially extendingannular opening 51 defined between radially inner and outer annularsurfaces. A seal ring 52 is located in an annular groove provided ininner annular surface and bears against the inner annular flange 48 ofthe movable member 44 to prevent exhaust gas flowing through the turbinewheel 14 via the cavity 50 rather than the inlet passageway 42. Asimilar ring seal 53 is provided between the outer flange 49 and thewall of the cavity 50.

The movable member 44 is supported on axially extending guide rods 54that are arranged so that they can reciprocate in the bearing housing11. An actuator 56 such as, for example, a motor or a pneumaticactuator, is operable to control the axial position of the guide rods54, and therefore the movable member 44, via a control linkage 55 in amanner which is well known in the art. By appropriate control of theactuator the axial position of the movable member 44 can be controlledso as to vary the size of the inlet passageway 42 between the radialwall 43 and the facing shroud plate 45.

The exhaust gas passage 35 is defined, as in the previous embodiment,through a wall of the bearing housing and the compressor back plate 27which is disposed between the rear of the compressor wheel 15 and thebearing housing 11. As before the passage 35 extends inwardly towardsthe shaft and emerges at a location between compressor end seals 18 (notshown in FIG. 3) that are disposed between the shaft 10 and the bearinghousing 11.

When the inlet passageway 42 is substantially closed by the movablemember 44, the flow of exhaust gas through the turbine 7 is restrictedthereby applying engine braking. As described in relation to theprevious embodiment the speed of rotation of the turbine wheel 14 isreduced and the compressor boost pressure drops correspondingly. Theapplication of engine braking exhaust gas pressure to the seal(s) 18thus prevents the pressure gradient across the seal or seals dropping toa level where there is a risk of oil passing from the bearing housing 11into the compressor housing 12.

Although not represented in the embodiment shown it is to be understoodthat a passageway equivalent to that (labelled 36) shown in FIG. 2 maybe provided at the turbine end to supply pressurised air to the turbineend seal 19 immediately after engine braking. This will serve to preventor restrict transient oil leakage into the turbine housing 13 when theoperation of the engine switches from engine braking mode to normalfired mode.

The two embodiments described above are applicable to an arrangementwhere only one seal 18, 19 is used between the bearing and compressor orturbine housings 11, 12, 13. In such an instance, the exhaust gas (air)is applied axially outboard of the respective seal to ensure there is apositive pressure gradient across the seal from the compressor/turbineside to the bearing housing side.

For a single seal embodiment at the compressor end the passage 35 mightemerge at a location that is radially spaced from the seal 18 and whichdirects the gas against the back face of the compressor impeller wheel15 from where it impinges on the compressor side of the seal 18. Forexample it might emerge at a location that its less than half or onequarter of the radial length of the outer periphery of impeller wheelfrom the axis. Similarly passage 37 might be arranged to direct airtowards the back face of the turbine wheel.

An alternative arrangement for reducing oil leakage in the turbochargerduring engine braking is shown in FIG. 4 in which the exhaust gaspressure generated during engine braking is used to reduce the internalpressure of the bearing housing 11 to achieve the same effect asdescribed above, i.e. to maintain a positive pressure differentialacross the seals 18, 19 from the compressor housing 12 to the bearinghousing 11 and from the turbine housing 13 to the bearing housing 11.The turbocharger 6 is not shown in detail again and it will beunderstood that the exemplary embodiments illustrated in FIGS. 2 and 3may be used in this arrangement. Reference numerals for componentscommon to the systems of FIGS. 1 to 3 are unchanged for convenience andease of understanding. As before the exhaust brake valve 9 is disposeddownstream of the turbine 7 and is operable to direct exhaust gas duringengine braking mode both to an outlet and to a bleed passage 60.

During operation of the turbocharged internal combustion engine 1,high-pressure combustion gases escape from between the pistons and thecylinders in which they reciprocate, into a crankcase 61 of the engine.These “blow-by” gases are vented to restrict the pressure in thecrankcase 61 as it will tend to build to such a magnitude that oil willbegin to leak past seals such as those around the crankshaft. Aventilation filter 62 is usually fitted at the vent to remove oil vapourand the gases are typically recirculated to the air inlet system of theengine 1 for combustion with the usual intake of air and fuel. For aturbocharged engine these crankcase ventilation (CCV) gases aretypically fed back into the air inlet path 3 upstream of theturbocharger or directly into the turbocharger through a cast boss inthe compressor inlet. In this instance the CCV gases are simply depictedas being vented to a conduit 63 extending downstream of the filter 62.The bleed passage 60 from the exhaust brake valve 9 is connected to theconduit 63 in the manner of a jet pump 64 that relies upon the venturieffect. For example, the exhaust gas (air) flows out of a nozzle 65 atthe end of the bleed passage and into the conduit 63 where it passes aventuri. The serves to accelerate the flow of the CCV gas and exhaustgas in the conduit and thereby to decrease the pressure in the conduit,including upstream of the jet pump 64. The pressure at the CCV vent isthereby reduced and as is the pressure in the crankcase 61. This in turnreduces the pressure at the oil sump in the crankcase 61 and, since theoil drain of the turbocharger bearing housing is connected to the oilsump in the crankcase, the pressure within the bearing housing isreduced such that the pressure differential across the seals 18, 19 ismaintained. The decrease in pressure in the bearing housing is likely tobe relatively low (e.g. 10-20 mbar) using this method but in manyinstances this will be sufficient to prevent the migration of oil alongthe shaft 10, past the seal(s) 18 and into the compressor housing 12.There may be a short delay in the pressure reduction being transmittedfrom the conduit 63 to the bearing housing 11 but since oil leakage isgenerally only occurs after prolonged periods of engine braking this isnot thought to be problematic.

Numerous modifications and variations to the embodiment described abovemay be made without departing from the scope of the invention as definedin the appended claims. For example, the exhaust brake valve 9 may becombined with the bleed valve 20 so that the opening of the bleed path17 is dependent on the operation of the exhaust brake valve 9. Theplenum chamber may be omitted or in some applications may only beprovided between the valve 20 and the turbine end seal(s) 19.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiments have been shown and described and thatall changes and modifications that come within the scope of theinventions as defined in the claims are desired to be protected. Itshould be understood that while the use of words such as preferable,preferably, preferred or more preferred utilized in the descriptionabove indicate that the feature so described may be more desirable, itnonetheless may not be necessary and embodiments lacking the same may becontemplated as within the scope of the invention, the scope beingdefined by the claims that follow. In reading the claims, it is intendedthat when words such as “a,” “an,” “at least one,” or “at least oneportion” are used there is no intention to limit the claim to only oneitem unless specifically stated to the contrary in the claim. When thelanguage “at least a portion” and/or “a portion” is used the item caninclude a portion and/or the entire item unless specifically stated tothe contrary.

The invention claimed is:
 1. An engine braking method for an internalcombustion engine having an air intake path and an exhaust gas path, aturbocharger comprising a turbine in the exhaust gas path and acompressor in the intake path, the turbine comprising a turbine wheelrotatably disposed in a turbine housing, the compressor comprising acompressor impeller rotatably disposed in a compressor housing, theturbine wheel and compressor impeller being connected by a turbochargershaft, a bearing housing disposed between the compressor and turbine forhousing a bearing assembly to support the turbocharger shaft inrotation, at least one first seal between a compressor end of the shaftand the bearing housing, the method comprising: operating the engine inan engine braking mode by activating an exhaust gas brake to impede theflow of exhaust gas in the exhaust gas path when the engine throttle isclosed, thereby generating an exhaust gas back-pressure in the exhaustgas path; diverting at least a portion of the exhaust gas during enginebraking mode from the exhaust gas path in order to increase the pressuregradient across the at least one first seal in the direction from thebearing housing to the compressor housing.
 2. An engine braking methodaccording to claim 1, wherein the diverted exhaust gas is directed to alocation between the at least one first seal and the compressor housingso as to raise the pressure in that location.
 3. An engine brakingmethod according to claim 2, wherein the exhaust gas is diverted througha passage in the turbocharger that emerges to a side of the at least onefirst seal that is closest to the compressor.
 4. An engine brakingmethod according to claim 3, wherein the passage is disposed, at leastin part, through the bearing housing.
 5. An engine braking methodaccording to claim 1, wherein the diverted exhaust gas passes through apassage in the turbocharger that emerges so as to direct exhaust gas ata back face of the compressor wheel.
 6. An engine braking methodaccording to claim 1, wherein the at least one first seal is disposed ina bore in a compressor back plate and the diverted exhaust gas isdirected through a passage in the back plate.
 7. An engine brakingmethod according to claim 1, wherein the at least one first sealcomprises at least one pair of axially spaced first seals and thediverted exhaust gas is directed to a space between the at least onepair of axially spaced first seals so as to raise the pressure betweenthe seals.
 8. An engine braking method according to claim 1, furthercomprising using an exhaust brake valve in the exhaust gas path to applythe exhaust gas brake.
 9. An engine braking method according to claim 1,further comprising using a variable geometry turbine to apply theexhaust gas brake, by moving a movable member of the turbine to reducethe size of an inlet passageway of the turbine so as to restrict theexhaust gas flow through the turbine and create the back-pressure. 10.An engine braking method according to claim 1, wherein the exhaust gasis diverted to a bleed path during engine braking mode, at least some ofthe gas being stored temporarily in a plenum chamber in the bleed pathand being permitted to flow out the plenum chamber immediately afteroperation of the engine in engine braking mode.
 11. An engine brakingmethod according to claim 10, further comprising closing the bleed pathupstream of the plenum chamber when the engine is not being operated inengine braking mode.
 12. An engine braking method according to claim 1,further comprising deactivating the exhaust gas brake at the end ofoperating the engine in an engine braking mode, and diverting exhaustgas to a location between at least one second seal disposed between aturbine end of the shaft and the bearing housing so as to increase thepressure gradient across that seal in the direction from the bearinghousing to the turbine housing.
 13. An engine braking method accordingto claim 1, wherein the pressure gradient is increased by delivering thediverted exhaust gas into a conduit in fluid communication with thebearing housing, the exhaust gas being passed through a jet pumparrangement so as to effect a decrease in pressure in the conduit, inthe pressure in the engine crankcase and therefore in the bearinghousing of the turbocharger.
 14. An engine braking method according toclaim 13, wherein the conduit is connected downstream of a crankcaseventilation port of the engine.
 15. An engine braking system comprising:an internal combustion engine with an air intake path and an exhaust gaspath; a turbocharger comprising a compressor for delivering pressurisedair to the air intake path and a turbine for receipt of the exhaust gasfrom the engine, the turbine comprising a turbine wheel rotatablydisposed in a turbine housing, the compressor comprising a compressorimpeller rotatably disposed in a compressor housing, the turbine wheeland compressor impeller being connected by a turbocharger shaft, abearing housing disposed between the compressor and turbine for housinga bearing assembly to support the turbocharger shaft in rotation, atleast one first seal between the shaft and a compressor end of thebearing housing; an exhaust gas brake in the exhaust gas path andoperable to effect engine braking, an exhaust gas bleed path extendingfrom the exhaust gas path to a location between the at least one firstseal and the compressor housing for diverting at least a portion of theexhaust gas from the exhaust gas path during engine braking.
 16. Anengine braking system according to claim 15, further comprising a valvein the exhaust gas bleed path operable to open or close the exhaustbleed path.
 17. An engine braking system according to claim 15, whereinthe exhaust gas bleed path is in fluid communication with a passage inthe turbocharger that emerges at said location between the at least onefirst seal and the compressor housing.
 18. An engine braking systemaccording to claim 17, wherein the passage is disposed, at least inpart, through the bearing housing.
 19. An engine braking systemaccording to claim 17, wherein there is provided a compressor back platebetween the compressor wheel and an interior of the bearing housing, theat least one first seal being disposed in a bore in the compressor backplate, the passage being defined at least in part in the back plate. 20.An engine braking system according to claim 15, wherein the exhaust gasbleed path is in fluid communication with a passage in the turbochargerthat is configured to direct exhaust gas at a back face of thecompressor wheel, which back face faces the bearing housing.
 21. Anengine braking system, according to claim 15, wherein the at least onefirst seal comprises at least one pair of axially spaced first seals,the exhaust bleed gas path extending to a space between the axiallyspaced first seals.
 22. An engine braking system according to claim 15,wherein the exhaust gas brake is an exhaust brake valve in the exhaustgas path downstream of the turbine wheel.
 23. An engine braking systemaccording to claim 15, wherein the exhaust gas brake is a variablegeometry turbine having an exhaust gas inlet passageway defined betweena movable member and a facing wall, the movable member being movabletowards and away from said facing wall so as to vary the size of theinlet passageway.
 24. An engine braking system according to claim 15,wherein there is provided a plenum chamber in the exhaust gas bleedpath, the chamber being configured for the temporary storage of exhaustgas.
 25. An engine braking system according to claim 15, furthercomprising at least second one seal between a turbine end of the shaftand the bearing housing, the exhaust gas bleed path also extending to alocation between the at least one second seal and the turbine housing.26. An engine braking system according to claim 25, wherein the exhaustbleed path is in fluid communication with a passage in the turbochargerthat emerges at said location between the at least one second seal andthe turbine housing.
 27. A powered vehicle having an engine brakingsystem according to claim
 15. 28. A method for reducing oil leakage in aturbocharger comprising a compressor for delivering pressurised air toan air intake path of an internal combustion engine, and a turbine forreceipt of the exhaust gas from the engine, the turbine comprising aturbine wheel rotatably disposed in a turbine housing, the compressorcomprising a compressor impeller rotatably disposed in a compressorhousing, the turbine wheel and compressor impeller being connected by aturbocharger shaft, a bearing housing disposed between the compressorand turbine for housing a bearing assembly to support the turbochargershaft in rotation, at least one first seal between a compressor end ofthe shaft and the bearing housing, activating an exhaust gas brake toimpede the flow of exhaust gas into the turbine and diverting at least aportion of the impeded exhaust gas flow in order to increase thepressure gradient across the at least one first seal in the directionfrom the bearing housing to the compressor housing.